Secreted from mucus and cellulose, these structures often comprise several layers of filters and can reach up to ten times their body length.

[1][5][n 1] More species were quickly discovered, with Oikopleura in 1830 providing the first evidence of the larvacean house, although its role in feeding wouldn't be understood until Eisen's discoveries in 1874.

As the larvae of ascidian tunicates don't feed at all,[8] the larvae of doliolids goes through their metamorphosis while still inside the egg,[9] and salps and pyrosomes have both lost the larval stage,[10] it makes the larvaceans the only tunicates that feed and have fully functional internal organs during their tailed "tadpole stage", which in Appendicularia is permanent.

The full development of Oikopleura dioica and the fate of its cell lineages have been well-documented, providing insight into larvacean anatomy.

The pharynx is equipped with an endostyle on its lower side, a specialized organ helping direct food particles inside.

[16] Appendicularia retains the ancestral chordate characteristics of having the pharyngeal spiracles and the anus open directly to the outside, and by the lack of the atrium and the atrial siphon found in related classes.

The tail of larvaceans contain a central notochord, a dorsal nerve cord, and a series of striated muscle bands enveloped either by epithelial tissue (oikopleurids) or by an acellular basement membrane (fritillarids).

To assist in their filter-feeding, larvaceans produce a test or "house" made of mucopolysaccharides and cellulose,[19] secreted from specialized cells termed oikoplasts.

[12] In larvae, surface fibrils are secreted by the epithelium prior to the differentiation of the oikoplasts, and have been suggested to play a part in the development of the first house, as well as the formation of the cuticular layer.

This specific niche of "mucous-mesh grazers" or "mammoth grazers" has been argued to be shared with thaliaceans (salps, pyrosomes and doliolids) — all using internal mucous structures —, as well as with sea butterflies, a clade of pelagic sea snails similarly using an external mucous web to catch prey, although through passive "flux feeding" rather than active filter-feeding.

Unlike all other known larvaceans, Oikopleura dioica shows separate sexes, which are distinguished on the last day of their life cycle through differing gonad shapes.

[30] Through their discarded, nutrient-rich houses — termed sinkers — and fecal pellets falling towards the deep seafloor, larvaceans transport large amounts of organic matter towards that region, constituting a significant component of marine snow.

In this way, larvaceans are believed to play a part in the missing plastic paradox, transporting microplastics through the water column and to the seafloor.

Already in the late 19th to early 20th century, it was hypothesized by Seeliger and later by Lohmann that Appendicularia diverged first from a free-swimming ancestral tunicate, with sessile forms evolving later in the sister lineage (often termed Acopa).

[32] More recently, microfossils covered in an organic coat found in vanadium-rich Cambrian black shales in South China have been suggested to be traces of early larvaceans in their houses, putatively termed "paleoappendicularians".

[34][35] Vetulicolians have also been argued to represent stem-group larvaceans by Dominguez and Jefferies, on the basis of synapomorphies comprising the reduction of the atria and of the gill slits, the position of the anus, and a 90° counter-clockwise torsion of the tail (as seen from behind) around the anterior-posterior axis.

[36] The extant species of the class are divided into three families based on both morphological and genomic criteria: Kowalevskiidae, Fritillariidae and Oikopleuridae.

Meanwhile, Kowalevskiidae is notable for lacking the heart and endostyle present in other families, the latter replaced by a ciliated groove without glandular cells.

The shape of the spiracles also differs: they appear as simple holes in Fritillariidae, long narrow slits in Kowalevskiidae, and tubular passages in Oikopleuridae.

The popularity of Oikopleura dioica as a model organism and its ease of cultivation have led to studies disproportionately focusing on this species' anatomy, and in situ observations on Bathochordaeus charon have been performed by the Monterey Bay Aquarium Research Institute.